Blending of polymers has given a new direction for developing novel
materials. It is an easy and inexpensive method of modifying various
properties of the polymers. Polymer blending is a widely used technique
to improve some physical properties of homopolymers. Blending of two
polymers may either result in miscible or immiscible system.
Characteristics and morphology of blends of polyvinylidene fluoride and
polymethylmethacrylate (PVDF/PMMA) have been investigated by various
techniques such as X-ray, Fourier transform infrared (FTIR), optical,
calorimetric, microscopic, and electric measurements [1-6]. PVDF and
PMMA are completely miscible in the melt and exhibit a lower critical
solution temperature around 330[degrees]C.

PVDF is a semicrystalline polymer which has drawn both scientific
and technological attention because of the useful pyroelectric [7] and
piezoelectric [8] properties. It is also one of the rare polymers that
exhibit diverse crystalline forms, having at least five phases known as
[alpha], [beta], [gamma], [delta], and [epsilon] phases [9-11]. The
polar phases [beta] and [gamma] are technologically the most interesting
because of its better pyroelectric and piezoelectric properties. PVDF is
of interest because of its typical fluoropolymer characteristics and its
miscibility with PMMA, which is commonly used as advanced material.
There have been several studies regarding the miscibility of PMMA and
PVDF [12-14].

Also, PMMA has received great attention due to its optical
properties and its possible use in nonlinear optics. PMMA is a plastic
widely used for its stiffness and clarity in various industrial
applications. It can be used as a good keeper for rare earth and garnets
which has wide technological applications [15]. The structure
characteristics of PMMA have been the subject of several investigations
[16-18].

PVDF/PMMA blends have been studied extensively, mainly in relation
to PVDF piezoelectric properties [19, 20]. Much attention has been paid
to problems such as miscibility of the amorphous phase, crystallization of PVDF in various phases, and molecular origin of PVDF/PMMA
interactions. Moreover, blending with PMMA was described as an original
way to force PVDF to crystallize into the piezoelectric phase, which is
thermodynamically unstable in pure material [6].

The aim of the present work was an attempt to investigate the
effect of addition of PMMA on the crystallization and morphology of PVDF
phases. Spectroscopic, thermal, and morphological methods are
particularly useful in such investigations. The results are demonstrated
by FTIR, X-ray, UV-visible, differential thermal analysis (DTA), and SEM
spectroscopy.

EXPERIMENTAL

Samples Preparation

PVDF pellet (SOLEF 1008, Solvay, Belgium) with average molecular
weight 5.3 x [10.sup.5] and PMMA with an average molecular weight of 1.2
x [10.sup.5] were used. Appropriate amounts of PVDF and PMMA were
dissolved in tetrahydrofuran (THF). After complete dissolution and at
suitable viscosity, the blend was prepared by casting onto a glass Petri
dish, then left to evaporate the solvent slowly. The resulting PVDF/PMMA
films were then dried in a vacuum oven at 60[degrees]C for 3 days to
ensure the removal of the solvent traces. The blends of PVDF/PMMA were
prepared in different weight concentration (100/0, 80/20, 60/40, 50/50,
60/40, 80/20, and 0/100). The thickness of the films was in the range of
110-140 [micro]m.

[FIGURE 1 OMITTED]

Measurements

X-ray diffraction scans were obtained using DIANO corporation-USA
equipped using Cu K[alpha] radiation ([lambda] = 1.540 [Angstrom], the
tube operated at 30 kV, the Bragg angle (2[theta]) in the range of
10-50[degrees], step size = 0.1 and step time 1 sec). The FTIR
measurements were carried out using the single beam FTIR spectrometer
(FTIR-430, Jascow, Japan). The FTIR spectra of the samples were obtained
in the spectral range of 2000-400 [cm.sup.1] with scanning speed of 2
mm/sec. Ultra violet and visible (UV/VIS) absorption spectra were
measured in the wavelength region of 200-900 nm using spectrophotometer (V-570 UV/VIS/NIR, Jasco, Japan). The DTA of the prepared films was
carried out using an equipment type (Shimadzu DTA-50) from room
temperature to 300[degrees]C at a heating rate of 10[degrees]C/min.

The morphology of the blends was characterized by scanning electron
microscope using (JEOL 5300, Tokyo, Japan), operating at 30 kV
accelerating voltage. Surface of the samples were coated with a thin
layer of gold (3.5 nm) by the vacuum evaporation technique to minimize
sample charging effects due to the electron beam.

RESULTS AND DISCUSSION

X-ray Diffraction

X-ray diffraction (XRD) scans was a useful tool to examine the
influence of PMMA contents on the crystalline structure of PVDF in the
samples. Figure 1 shows the XRD scans of pure PVDF, pure PMMA, and their
blends. The interplanar distance (d values), 2[theta], and the
assignments of all the major peaks are listed in Table 1. The following
general observations are made on the basis of Fig. 1 and Table 1.

From Fig. 1, it is clear that for pure PVDF, the diffraction peak
observed at 2[theta] = 23.5[degrees], corresponding to the plans (101)
is due to crystalline structure of PVDF [beta]-phase. The diffraction
peaks at 2[theta] = 21.8[degrees] and 45.7[degrees], corresponding to
the plans (110) and (220), are due to crystalline structure of PVDF
[alpha]-phases. Pure PMMA exhibit an amorphous feature which is
characterized by two amorphous halos (a large hump) centered at 2[theta]
= 16[degrees] and a small hump at 2[theta] = 35[degrees] with no sharp
peaks.

From the diffraction scans and with increase PMMA content, it is
found that: (i) There is a decrease progressively in the relative
intensity (without disappear) of the peak appearing at 23.5[degrees],
i.e. the blends also exhibit [beta]-crystals of PVDF, (ii) The amorphous
halo becomes more and more obvious, this gives a clear indication of
complexation of the two polymers blend, i.e. reduces the long-rang order
in PMMA [21], and (iii) For the blends in which the PMMA was higher than
80 wt%, a broad peak near at about 45.7[degrees] was characteristic of
the combined (201 and 111) reflections of the [beta]-phase resulting
from molecular defects caused by head-head and tail-tail sequences [22].
However, as the PMMA content was higher than 80 wt% of PMMA, this
diffraction peak disappeared. Thus XRD analysis reveals that the blends
take place based on the influence of PMMA content on PVDF blends.

[FIGURE 2 OMITTED]

There was decrease in the relative intensity (height of the peak)
of the apparent peaks (2[theta] = 23.5[degrees]) with increasing PMMA
content. These results can be interpreted by considering by the Hodge et
al. [23] criterion which establishes a correlation between the height of
the peak and the degree of crystallinity.

FTIR Analysis

Figure 2 shows FTIR vibrational spectra of PVDF/PMMA films blend
from 2000 to 400 [cm.sup.1]. Characteristic absorption bands were
identified in these spectra and assigned by comparison with the
literature values found for PVDF, PMMA, and their blends. Our
assignments for these spectra are listed in Tables 2 and 3. According to
characteristic absorption bands of [alpha]-phase at (484, 1074, 1257,
and 1729 [cm.sup.1]), [beta]-phase at (512, 840, 877, and 1409
[cm.sup.1]) for pure PVDF are indicated in Fig. 2 [24, 25]. The
vibrational band at 512 [cm.sup.1] corresponds to bending vibrations
mode of C[F.sub.2] dipoles, characteristic of TT (trans) conformation of
the ferroelectric [beta]-phase of PVDF.

The vibrational bands at 987 and 1455 [cm.sup.1] are assigned to
O-C[H.sub.3] bending and stretching deformation of PMMA. The bands at
1712 and 1250 [cm.sup.1] are assigned to stretching frequency of C=O of
PMMA. The absorption band appearing at 854 [cm.sup.1] is assigned to the
characteristic frequency of vinylidene compound. It is clear that the
stretching frequency at 1712 [cm.sup.1] which corresponds to C=O of pure
PMMA, is shifted to 1729 [cm.sup.1] in the blends. This shift observed
in the carbonyl stretching frequencies of blends when compared to pure
PMMA is due to specific interaction between the carbonyl groups of PMMA
and the C[H.sub.2] groups of PVDF and indicates the formation of blends.
This is agreement with results reported by Colemann and Painter [26] for
PMMA/PVDF binary blends.

To investigate the influence of the PMMA chains on the formation of
ferroelectric crystalline [beta]-phase of PVDF, we have plotted the
intensity corresponding to the band at 877 [cm.sup.1] as a function of
PMMA content. As seen in Fig. 3, it gradually decreases as PMMA is added
to PVDF.

UV-Visible Analysis

UV-visible absorption spectra of PMMA/PVDF blend are shown in Fig.
4. The absorption edges were observed around 240 to 290 nm. The sharp
absorption edge for PVDF indicates the semicrystalline nature of PVDF. A
shift in band edges toward the higher wavelengths with different
absorption intensity for PMMA doped PVDF was observed. These shifts
indicate the formation of inter/intra between PMMA and PVDF that are in
consistence with X-ray, FTIR, and DTA results. Also, the shift in
absorption edge in the films reflects the variation in the optical
energy band gap, [E.sub.g].

It is clear that some blends exhibit a well-defined window of
wavelength range 290-350 nm. A sharp and a maximum height of this window
are noticed at the blend (50/50). The present optical window can be used
as an optical sensor or band pass filter for the wavelength range
290-350 nm in the UV and VIS regions.

[FIGURE 3 OMITTED]

The relation between the absorption coefficient [alpha] and the
optical energy band gap obeys the classical Tauc's expression. The
optical energy band gap for an indirect transition can be determined by
using the following relation [27]:

[E.sub.g] = hv - ([alpha]hv/b)[.sup.1/2] (1)

where h is plank's constant, v is the photon frequency, B is a
constant, and [alpha] is the absorption coefficient, which can
determined as a function of photon frequency using the equation:

[alpha] = [A/d] x 2.303 (2)

where A is the absorbance and d is the thickness of the sample. The
plot of ([alpha]hv)[.sup.1/2] versus the photon energy hv at room
temperature shows a linear behavior which are presented in Fig. 5a and
b. Each linear portion indicates an optical band gap, [E.sub.g], which
can be considered as an evidence for indirect allowed transition. Figure
6 displays the PMMA content dependence of [E.sub.g]. It is clear that
the optical energy band gap decreases with increasing PMMA content. The
existence and variation of optical energy gap, [E.sub.g], may be
explained by invoking the occurrence of local crosslinking within the
amorphous phase of PMMA and PVDF.

Differential Thermal Analysis

The thermal behavior of PVDF/PMMA blends was studied using DTA from
room temperature to 300[degrees]C. DTA thermograms (after smoothing) are
shown in Fig. 7. A sharp endothermic peak is attributed to the melting
temperature ([T.sub.m]). In DTA thermograms, the original PVDF showed a
single melting peak at 167.11[degrees]C which could be ascribed to the
presence of [alpha]-phase crystals of PVDF, according to the results of
FTIR and X-ray studies. It can be seen that the melting temperature of
PVDF/PMMA blends depresses markedly with increasing PMMA content. For
example, the [T.sub.m] of PMMA/PVDF (80/20) blends become
150.91[degrees]C, being 16.2[degrees]C lower than that of pure PVDF.

The area under crystalline melting endothermic peaks is correlated
to the degree of crystallinity. If it is assumed that all PVDF crystal
form present has the same heat of fusion [DELTA][H.sub.c] = 104.7 J/g
for completely crystalline PVDF sample [28], then [DELTA][H.sub.f]
(measured directly from DTA thermograms) can be related to the
crystallinity of the sample by the following equation:

Crystallinity = [[[DELTA][H.sub.f]]/[[DELTA][H.sub.c]]] x 100 (3)

The calculated values of the degree of crystallinity and the values
of the melting temperature of the blends from DTA thermograms are listed
in Table 4.

It is observed that, the degree of crystallinity and the melting
temperature for the blends are reduced with increasing PMMA content.
These results data accords with FTIR and X-ray finding. The dramatic
decrease in the crystallinity and the melting is related to a good
miscibility between PVDF and PMMA. Such is legitimate in that PMMA is
largely amorphous and does not contribute to the heat of fusion.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Morphology

To investigate fully the effect of PMMA content, the morphology of
PMMA/PVDF blends were studied using scanning electron microscope (SEM).
Figure 8a-g shows the SEM micrographs of the surface for PMMA/PVDF
blends at magnification 10,000 times. The micrograph in Fig. 5a for pure
PVDF is characterized by normal crystalline domains uniformly shaped
with equal size. The presence of PMMA leads to changes in the surface
morphology (Fig. 8b-g). The blend in Fig. 8b gave rise to crystalline
domains with coarse spherulitic structure. From these micrographs it can
be seen that the spherulites will increase with the addition of PMMA, up
to 60 wt% of PMMA, which gives rise to extended crystalline regions, as
shown in Fig. 8e. This is due to the fact that the PMMA segregated into
interlamellar or intercrystalline regions of PVDF. These spherulites are
a very useful feature for piezoelectric and other applications.

[FIGURE 6 OMITTED]

PMMA/PVDF 80/20 as shown in Fig. 8f have distinct longitudinal
shape not spheres, and the spherulites disappear. These distinct
longitudinal shapes has been attributed to a strong increase of lamellar twisting period and to a decreased radial growth rate in amorphous
regions with higher PMMA content [29]. There are small differences that
are visible on micrographs of PVDF/PMMA blend containing 80 wt% of PMMA
compared to morphology of virgin PMMA in Fig. 8g. No crystalline
structure is clearly observed on the surface of virgin PMMA and the
surface is rather rough. This is very similar to that observed by
Omastova and Simon [30].

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

CONCLUSIONS

XRD analysis reveals that the blends take place based on the
influence of PMMA content on PVDF blends. Characteristic absorption
bands from FTIR spectrum were identified and assigned by comparison with
the literature values found for PVDF/PMMA blends. The shift of C=O
observed in the carbonyl stretching frequencies of blends is due to
specific interaction between the carbonyl groups of PMMA and the
C[H.sub.2] groups of PVDF. The change in the UV-visible spectrum is due
to complex formation which can be reflected in the form of decrease in
the optical energy gap. The DTA thermograms depicts that the addition of
PMMA decreased the melting temperature and the degree of crystallinity.
Morphology of PMMA/PVDF blends shows crystalline domains uniformly
shaped with spherulites. The morphology becomes sharper and contains a
longitudinal shape note spheres for PMMA/PVDF (80/20).